This invention relates to a liquid crystal display device and more particularly to a liquid crystal display device which possesses a highly desirable viewing angle property, a high contrast ratio, and excellent display quality.
In recent years, liquid crystal display devices have been finding extensive utility as display devices in television sets or graphic displays by virtue of such features as thin volume and low electric power consumption. Meanwhile, the necessity of developing liquid crystal display devices which feature high contrast and possess wide viewing angles has been felt acutely.
As respects the manner of display adopted in these liquid crystal display devices, the twist nematic (TN) type which uses a nematic liquid crystal as a liquid crystal composition and the super twisted nematic (STN) type which is an applied version of the TN type have been prevailing to date.
The principle of display according to the TN type will be explained below with reference to FIG. 1A and FIG. 1B is a schematic diagram which is intended to aid in the illustration of the principle of display by the TN type.
The orientation layer in the upper substrate and that in the lower substrate are so treated that the directions of orientation of the liquid crystals in the upper and lower substrates will perpendicularly intersect each other. In this case, as shown in FIG. 1A, the liquid crystals in the display device exhibit an optical rotatory power of 90.sup..degree. because their average directions of major molecular axes are oriented as twisted by 90.degree. .
When this display device is interposed between two mutually parallel polarizing elements, the light is blocked owing to the optical rotatory power of liquid crystals. When voltage is applied to the device, the light is passed through the liquid crystals because the liquid crystals are erected upright except for the areas in the immediate neighborhood of the orientation layers and consequently are deprived of the optical rotatory power as shown in FIG. 1B.
Though the STN type display has the same liquid crystal cell structure as the TN type display, it requires to set the angle of twist of the direction of the average major molecular axis of the liquid crystals in the display device at about 270.degree. C. This display relies for its operation on a change in double refraction and not on a change in optical rotatory power to be brought about by a change in liquid crystal configuration which is induced by application of voltage. This display permits a multiplexed drive on about 200 scanning lines.
The liquid crystal display devices operated by the TN type and the STN type have problems on display characteristics which arise necessarily from their own principles of display. When the screens of the liquid crystal display devices of these two types of display are looked at from an oblique direction, the directions in which the rise of liquid crystal molecules is observed during the application of a voltage are uniquely decided by the viewing angle. Though the screens observed from these directions appear in high contrast, they succumb to color inversion during the display of a halftone image. The screens observed from the directions inverted by 180.degree. from those mentioned above appear in low contrast. To be specific, these types of display both encounter the problem that the viewing angles in which displays in the screens are observed to advantage are limited in a narrow range. Thus, studies are being continued in search of a measure to widen the range imposed on the viewing angles.
In the first place, attempts to equalize the ranges of viewing angle in the methods of TN type and STN type display have been proposed to date. For example, JP-A-59-204,822, JP-A-59-204,824, JP-A-60-147,722, etc. have disclosed a method which obtains the desired increase of the range by giving a concentric rubbing treatment to both the upper and the lower substrate and changing the position of center of rotation between the upper and lower substrates and JP-A-60-256,120 and the like have disclosed a method which accomplishes the increase of the range by giving a concentric rubbing treatment to either of the two substrates and a radial rubbing treatment to the remaining substrate.
These attempts to equalize the ranges of viewing angle in the TN type and STN type display, however, reside invariably in performing a treatment of orientation on the entire surfaces of the substrates. Since the area of disturbed orientation in the neighborhood of the center of orientation (the center of concentric rings or the center of radiation) has a size of the level of an pixel, the pixel within the area of disturbed orientation has the possibility of encountering difficulty in effecting display as expected. Besides, the area of disturbed orientation is at a disadvantage in allowing no improvement in contrast.
A display with a polymer dispersion type liquid crystal has been also proposed. This polymer dispersion type operates on the principle that the display is attained by repressing the scattering of light in the polymer dispersion type liquid crystal display panel thereby controlling the light transmission through the display panel. Unlike the TN type display, therefore, this type avoids such adverse characteristics of viewing angle as low screen contrast inevitably arising from the relation between the direction of orientation of liquid crystal molecules and the direction of observation. It has been demonstrated to be capable of realizing a display property allowing uniform contrast throughout the entire range of viewing angle.
This polymer dispersion type liquid crystal display, however, operates on the principle that a dark state is obtained by scattering the light in transmission. It is, therefore, at a disadvantage in obtaining no sufficient contrast as compared with the TN type display because it is difficult to lower the level of the dark state.
Moreover, the liquid crystal displays discussed above invariably suffer from poor utility of light and, as a result, permit no sufficient decrease in the transmittance in the display of black. In the display of white, they are at a disadvantage in giving only low contrast because the transmittance is not sufficiently improved.